Depending on the point in the game when the fort is approached, the exterior of the fort may be defended by five machine gun turrets that are placed on its roof.
There are two ways of entering the fort: via the roof or via the garage. Of the two, the garage is the route that bypasses the most enemies (the synths on the second floor, and the machine gun turrets if present), but this path also bypasses notable loot. The garage is direct to the southwest of the main entrance. A chemistry station and a bed can be found in the garage, to the south of the door leading to the interior of Fort Hagen. The roof entrance can be reached by going around to the west side of the building and climbing the scaffolding. A hatch on the eastern roof will allow the player character to enter the building.
The entire interior of the fort is filled with synths. On the upper floor, upon entering the building from the roof, the player will find a kitchen to the east with an issue of Guns and Bullets on the table. Heading north into the hallway, the room to the east will have a cooking station and a hole connecting it to another room occupied by a machine gun turret. West of the hallway is a room mostly empty save for two Nuka Cola machines. Both rooms connect to another room which has a hole in the floor and also leads to the staircase. Both lead to the second floor.
A protectron can be found and activated in the stairwell of the second floor. It, however, will not attack synths but instead may turn against the player. The second floor wraps around the staircase. A chained door to the west prevents immediate progress in that direction, so one must loop around the floor eastwards. The room to the southwest of the second floor will contain a machine gun turret. Continuing west and then north leads to an advanced locked terminal with an active mine lying under it and a military trunk with ammo and weapons next to it. The console will allow deactivation of the nearby laser turret and open the magnetic door further back in the room. Across from the magnetic door, the player can unchain the door heading back to the stairwell.
If the Reunions section of the main story line quest is active, or has been completed, an elevator will be found at the back of the room, leading onward to the basement. If the Reunions quest has not yet been reached, the elevator is blocked by wooden bars.
From the elevator, the player is confronted with a passage with a room at the end occupied by a machine gun turret. The passage has two doors on its east side. The first one can be unblocked from this side and is the door on the ground floor which leads to the garage entrance and the stairwell. In between the first and second door is a Tesla trap. The second door leads to a room with armor and weapons workbenches on the south and east walls. In the room at the end of the passage is a staircase heading down. Behind the security gate is another Tesla trap.
Heading down another set of stairs leads to another security gate, this one guarded by a machine gun turret. At the end of the hall is a door leading to the Fort Hagen Command Center; until the Reunions quest becomes active the door is blocked by a plywood barrier.
| ||For notable loot in the Command Center, see Fort Hagen Command Center.|
- Guns and Bullets - On a table in the kitchen in the southeast corner of the fort. To the right upon entering the fort through the rooftop entrance.
- FLL3 turbopump bearings - If the quest Last Voyage of the U.S.S. Constitution is active, Fort Hagen may be selected from among a list of radiant locations to find the part. The bearings can be found inside the green trunk in the same room where elevator to basement exist.
- The command center is only accessible after the associated quest Reunions becomes active; the elevator that leads to it will be boarded up and unusable otherwise.
- Judging by the warehouses at the base and the presence of armored vehicles and multiple power armor stations, it seems Fort Hagen was home to units from one of the US Army armored divisions.
- When on the quest Reunions, and using Dogmeat to track, upon arriving at Fort Hagen he may sit in front of the building's main doors, and not move despite commands to follow. To fix this, complete the mission and he should follow the Sole Survivor again.
- The areas to the north, northeast, and west of Fort Hagen can have some potentially very dangerous enemies spawn randomly, they include but are not limited to: a sentry bot, a super mutant behemoth and/or a patrol of leveled gunners that are possibly joined by a Gunner commander wearing power armor.
- The announcer in the elevator will only ever say, "Going up," although the elevator actually cycles between three floors. The floors are announced, but are garbled randomly as "Bwaaarbub Floor," "Kzztth Floor" and "POP zzt Kthlor."
Fort Hagen appears only in Fallout 4.